Anode For A Cell Of A Lithium-Ion Battery, Its Manufacturing Process And The Battery Incorporating It

ABSTRACT

An anode usable in a cell of a lithium-ion battery comprising an electrolyte based on a lithium salt and a non-aqueous solvent, to a process for manufacturing this anode and to a lithium-ion battery having one or more cells incorporating this anode. This anode is based on a polymer composition, obtained by melt processing and without solvent evaporation, that is the product of a hot compounding reaction between an active material and additives having a polymer binder and an electrically conductive filler. The binder is based on at least one crosslinked elastomer and the additives furthermore include at least one non-volatile organic compound usable in the electrolyte solvent, the composition advantageously includes the active material in a mass fraction greater than or equal to 85%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/797,004 filed Mar. 12, 2013, now U.S. Pat. No. 9,082,999, which claims priority to French Application No. 12 52252 filed Mar. 13, 2012, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to an anode that can be used in a cell of a lithium-ion battery, to a process for manufacturing this anode, and to a lithium-ion battery having one or more cells incorporating this anode.

BACKGROUND

There are two main types of lithium storage battery: lithium-metal batteries, where the negative electrode is made of lithium metal (which material causes problems with safety when in the presence of a liquid electrolyte), and lithium-ion batteries, where the lithium remains in the ionic state.

Lithium-ion batteries consist of at least two conductive Coulombic electrodes of different polarities, the negative electrode or anode and the positive electrode or cathode, between which electrodes a separator is located, which separator consists of an electrical insulator imbibed with an aprotic electrolyte based on Li⁺ cations ensuring the ionic conductivity. The electrolytes used in these lithium-ion batteries usually consist of a lithium salt, for example of formula LiPF₆, LiAsF₆, LiCF₃SO₃ or LiClO₄, which is dissolved in a mixture of non-aqueous solvents such as acetonitrile, tetrahydrofuran, or more often a carbonate, for example of ethylene or propylene.

The active material of the anode of a lithium-ion battery is typically graphite (capacity of 370 mAh/g and redox potential of 0.05 V relative to the Li⁺/Li couple) or, as a variant, mixed metal oxides, among which are included the lithiated titanium oxides of formula Li₄Ti₅O₁₂, Li_(x)TiO₂, or even compounds, for example, of formula Li_(x)CuVO₄, Li_(x)SnO₂, Li_(x)MoO₂, Li_(x)MoS₂, where 0<x<5. The reversible insertion/removal of lithium into/from these active anode materials takes place over electrochemical potentials that are generally higher than that of graphite.

The active material of the cathode of a lithium-ion battery is conventionally an oxide of a transition metal, such as an oxide of vanadium, nickel, manganese or cobalt, or, as a variant, it may be a lithiated iron phosphate.

The respective active materials of the cathode and anode of a lithium-ion battery allow reversible insertion/removal of lithium into/from these electrodes, and the higher the mass fraction of the active materials in the electrodes, the higher their capacity. These electrodes must also contain an electrically conductive compound, such as carbon black and, in order to provide them with sufficient mechanical cohesion, a polymer binder. A lithium-ion battery is thus based on the reversible exchange of lithium ions between the anode and the cathode during the charging and discharging of the battery, and, for a very low weight, by virtue of the physical properties of lithium, such a battery has a high energy density.

The anodes of lithium-ion batteries are most often manufactured using a process comprising, in succession, a step of dissolving or dispersing the various ingredients of the anode in a solvent, a step of spreading the obtained solution or dispersion on a metallic current collector, and then lastly a step of evaporating this solvent.

Processes for manufacturing the anodes of lithium-ion batteries that use an organic solvent have many drawbacks with respect to the environment and safety. In particular, it is, in this case, necessary to evaporate large amounts of such solvents, which are toxic or inflammable.

As for processes that use an aqueous solvent to manufacture these anodes, their major drawback is that the anode must be very thoroughly dried before they can be used, traces of water being known to limit the useful lifetime of lithium storage batteries. Mention may, for example, be made of document EP-B1-1 489 673, which describes a process for manufacturing an anode based on graphite and an elastomer binder, and using an aqueous solvent.

It is therefore highly desirable, for lithium-ion batteries, to prepare anodes that are manufactured without using solvents. It is in this context that processes for manufacturing anodes for lithium-ion batteries using melt processing techniques (for example extrusion) have been described in the literature.

Unfortunately, these melt processes cause major difficulties in the case of lithium-ion batteries, which, as is known, require a mass fraction of active material in the polymer mixture of the anode of at least 85% for the latter to have sufficient capacity in the lithium-ion battery. However, at such contents of active material the viscosity of the anode polymer mixture becomes very high, and leads to a risk of the mixture over-heating and losing its mechanical cohesion once it is in use.

Document U.S. Pat. No. 6,939,383 describes the extrusion of a polymer composition comprising a poly(ethylene oxide)-poly(propylene oxide)-poly(glycidyl ether) copolymer by way of ionically conducting polymer, for solventless processing of an anode or cathode for a lithium-polymer polymer battery. However, the mass fraction of active material in the single polymer composition prepared in this document is only 64.5% and, furthermore, it relates to a cathode.

Document U.S. Pat. No. 5,749,927 discloses a process for the continuous preparation of lithium-polymer batteries by extrusion, which process comprises compounding the anode or cathode active material with an electrical conductor and a solid electrolyte composition comprising a polymer, a lithium salt and a propylene carbonate/ethylene carbonate mixture greatly in excess over this polymer. In this document, the mass fraction of active material present in the only anode polymer composition obtained is also below 70%.

Thus, a major drawback of these known melt processes for manufacturing anodes for lithium storage batteries is that the mass fractions of active material in the anode polymer composition are insufficient to obtain high-performance anodes specifically for lithium-ion batteries.

SUMMARY

One aim of the present invention is therefore to provide a process, for manufacturing an anode, that overcomes all of the aforementioned drawbacks, and this aim is achieved since the Applicant has discovered, surprisingly, that if an active material and additives comprising a crosslinkable elastomer matrix, an electrically conductive filler and a non-volatile (i.e. having a boiling point above 150° C. at an atmospheric pressure of 1.013×10⁵ Pa) organic compound, are hot compounded by melt process and without solvent evaporation, then an anode polymer composition is obtained, after crosslinking, that is usable in a lithium-ion battery comprising an electrolyte based on a lithium salt and a non-aqueous solvent, with a fraction of this active material in the composition clearly higher than those obtained previously by melt processing, and advantageously greater than or equal to 85%, the one or more organic compounds advantageously being used as a solvent for the electrolyte.

An anode according to the invention, usable in a cell of a lithium-ion battery comprising an electrolyte based on a lithium salt and a non-aqueous solvent, is thus based on a polymer composition, obtained by melt processing and without solvent evaporation, that is the product of a hot compounding reaction between an active material and additives comprising a polymer binder and an electrically conductive filler, and the anode is such that the binder is based on at least one crosslinked elastomer and such that these additives furthermore comprise at least one non-volatile organic compound that is usable in this electrolyte solvent, the composition comprising the active material in a mass fraction advantageously greater than or equal to 85%.

Even more advantageously, said composition may comprise said active material in a mass fraction of 90% or more, or even 93% or more.

It will be noted that this very high mass fraction of active material in the anode according to the invention ensures that the or each cell obtained is a high-performance cell and therefore that the lithium-ion battery incorporating them is a high-performance battery.

It will also be noted that the uniform distribution of said at least one crosslinked elastomer in the composition ensures the mechanical strength of the anode.

It will furthermore be noted that an anode according to the invention is totally water-free, in contrast to those of aforementioned document EP-B1-1 489 673.

Advantageously, said active material may comprise at least one compound or complex that is chosen from the group consisting of graphite, lithiated titanium oxides, the lithiated compounds of formula Li_(x)CuVO₄, Li_(x)SnO₂, Li_(x)MoO₂ and Li_(x)MoS₂ (where 0<x<5), and the lithiated metal sulphides of formula LiVS₂ or LiTi₂S₂, and that is preferably chosen from the lithiated titanium oxides of formula Li₄Ti₅O₁₂ or Li_(x)TiO₂.

It will be noted that, advantageously, the lithiated titanium oxide Li₄Ti₅O₁₂, which has a capacity of 170 mAh/g, has a very high cyclability, due to the fact that its volume does not change greatly during charge/discharge cycles (i.e. cycles of insertion/removal of Li⁺), a capacity to withstand high rates, and a high thermal stability. In addition, by virtue of its high redox potential of 1.5 V, relative to the Li⁺/Li couple, this lithiated titanium oxide ensures that the electrolytes used exhibit a satisfactory stability when in contact with the anode that incorporates it, no solid interphase forming between the anode and the electrolyte.

Preferably, said at least one elastomer is a peroxide-crosslinked diene elastomer and, even more preferably, a hydrogenated nitrile rubber (HNBR). Also preferably, said at least one elastomer may be present in said composition in a mass fraction between 1% and 10%.

Advantageously, said at least one non-volatile organic compound may comprise a carbonate, preferably a carbonate of at least one olefin such as ethylene, which is preferably used in the electrolyte composition.

It will be noted that using such a carbonate, such as an ethylene carbonate, advantageously allows:

-   -   the filler content in the composition to be increased;     -   the inherent risks with respect to the toxicity of volatile         organic compounds (VOC), used in conventional processes for         manufacturing anodes, to be avoided because this carbonate is a         product that is solid at room temperature and much less         hazardous to handle; and     -   the anode polymer composition to be used without evaporating the         carbonate beforehand, and the incorporation of the electrolyte         into the anode to be made easier because this carbonate is one         of the main constituents of electrolytes used at the present         time in lithium-ion batteries.

Also advantageously, said at least one organic compound may be present in said composition in a mass fraction between 0.1% and 5%.

Preferably, the mass ratio of the volatile organic compound(s)/crosslinked elastomer of the binder is lower than 1.

It will be noted that the invention allows the salts required for operation of the anode to be incorporated during its manufacture.

According to another feature of the invention, said additives may furthermore comprise a crosslinking system that is present in the composition in a mass fraction between 0.05% and 0.20%, and that preferably comprises an organic peroxide and a crosslinking co-agent in the case where said at least one elastomer is a diene elastomer such as a hydrogenated nitrile rubber (HNBR).

According to another feature of the invention, said electrically conductive filler may be chosen from the group consisting of carbon black, graphite, expanded graphite, carbon fibres, carbon nanotubes, graphene and their mixtures, and is present in the composition in a mass fraction between 1% and 6%.

A process, according to the invention, for manufacturing an anode such as defined above, is characterized in that it comprises:

a) melt compounding, without solvent evaporation, in an internal mixer or an extruder, of said active material and said additives comprising said binder and said organic compound in the solid state in order to obtain said composition in the crosslinkable state, this active material preferably comprising at least one compound or complex such as graphite or a lithiated titanium oxide; and

b) crosslinking and optionally hot forming this composition, in order to obtain said crosslinked composition.

According to another feature of the invention, step a) may be carried out by compounding said binder into a powder premix of the other ingredients of the composition, for example at a temperature between 80° C. and 120° C. in an internal mixer.

According to another feature of the invention, step b) may be carried out by hot pressing said crosslinkable composition.

Advantageously, the process of the invention may then comprise a step c) of rolling said crosslinked composition so as to deposit it on a metallic current collector with which said anode is equipped.

A lithium-ion battery according to the invention comprises at least one cell comprising an anode such as defined above, a cathode, and an electrolyte based on a lithium salt and a non-aqueous solvent.

According to another advantageous feature of the invention said electrolyte solvent may comprise said at least one non-volatile organic compound of the anode.

According to another aspect of the invention, said anode comprises a metallic current collector making contact with at least one film made of said polymer composition.

Advantageously, the cathode of this battery may be based on an active material comprising at least one lithiated polyanionic compound or complex having an operating voltage below 4 V and preferably carbon coated, such as a lithiated metal M phosphate of formula LiMPO₄, where M is for example an iron atom. Even more advantageously, this cathode may be obtained according to the process described in patent application FR1 250 457 filed on 17 Jan. 2012 in the name of the Applicant.

Other features, advantages and details of the present invention will become clear from reading the following non-limiting description of several exemplary embodiments of the invention, given by way of illustration.

DETAILED DESCRIPTION Example 1

An anode polymer composition was prepared in a Haake internal mixer at 90° C., the composition having the following formulation expressed in mass fraction (%):

HNBR binder (“Therban 4307”) 2.82 Carbon black 2.72 Ethylene carbonate 0.52 Active material Li₄Ti₅O₁₂ 93.84 Crosslinking system: Dicumyl peroxide 0.04 Triallyl cyanurate (TAC) 0.05

The various compounds were introduced into this internal mixer in succession, starting with the hydrogenated nitrile rubber by way of a crosslinkable diene elastomer (HNBR binder), and then a premix in powder form of the other ingredients above. After this compounding, and hot pressing at 170° C. for 15 minutes simultaneously allowing the binder to be crosslinked, several electrodes having a thickness ranging from 0.4 mm to 2 mm, each able to form an anode inside a cell of a lithium-ion battery, after deposition on a current collector with which this anode is equipped, were obtained directly.

Example 2

Another anode polymer composition based on the same ingredients as used in example 1 was prepared in a Haake internal mixer at 110° C., the composition however having the following different formulation expressed in mass fraction (%):

HNBR binder (“Therban 4307”) 8.23 Carbon black 4.15 Ethylene carbonate 1.64 Active material Li₄Ti₅O₁₂ 85.69 Crosslinking system: Dicumyl peroxide 0.12 Triallyl cyanurate (TAC) 0.15

The various compounds were introduced into this internal mixer in succession, starting with the hydrogenated nitrile rubber by way of a crosslinkable diene elastomer (HNBR binder), and then a premix in powder form of the other ingredients above. After this compounding, and hot pressing at 170° C. for 15 minutes simultaneously allowing the binder to be crosslinked, several electrodes having a thickness ranging from 0.4 mm to 2 mm, each able to form an anode inside a cell of a lithium-ion battery, after deposition on a current collector with which this anode is equipped, were obtained directly.

Example 3

Another anode polymer composition based on an active material other than that used in examples 1 and 2, and on the same other ingredients used in these examples, was prepared in a Haake internal mixer at 110° C., the composition having the following different formulation expressed in mass fraction (%):

HNBR binder (“Therban 4307”) 5.17 Carbon black 2.28 Ethylene carbonate 1.19 Active material (“Timrex KS 6L” graphite) 91.17 Crosslinking system: Dicumyl peroxide 0.08 Triallyl cyanurate (TAC) 0.11

The various compounds were introduced into this internal mixer in succession, starting with the hydrogenated nitrile rubber by way of a crosslinkable diene elastomer (HNBR binder), and then a premix in powder form of the other ingredients above. After this compounding, and hot pressing at 170° C. for 15 minutes simultaneously allowing the binder to be crosslinked, several electrodes having a thickness ranging from 0.4 mm to 2 mm, each able to form an anode inside a cell of a lithium-ion battery, after deposition on a current collector with which this anode is equipped, were obtained directly.

It will be noted, with reference to these exemplary embodiments of the invention, that the very high mass fraction (higher than 85%, indeed even higher than 90%) of the active material in this anode ensures that the or each cell thus obtained is a high-performance cell and therefore that the lithium-ion battery incorporating them is a high-performance battery. 

1. Process for manufacturing an anode according to claim 1, wherein said method comprises: a) melt compounding, without solvent evaporation, in an internal mixer or an extruder, of said active material and said additives comprising said binder and said organic compound in the solid state in order to obtain said composition in the crosslinkable state, this active material preferably comprising at least one compound or complex such as graphite or a lithiated titanium oxide; and b) crosslinking and optionally hot forming this composition, in order to obtain said crosslinked composition.
 2. Manufacturing process according to claim 1, wherein step a) is carried out by compounding said binder into a powder premix of the other ingredients of the composition, for example at a temperature between 80° C. and 120° C. in an internal mixer.
 3. Manufacturing process according to claim 1, wherein step b) is carried out by hot pressing said crosslinkable composition.
 4. Manufacturing process according to claim 1, wherein the method then comprises a step c) of rolling said crosslinked composition so as to deposit it on a metallic current collector with which said anode is equipped. 